Austin, Texas--Nearby massive black holes may be wearing shrouds of dust, but they aren't dead yet. As many as 10% of the black holes in neighboring galaxies are still gobbling up gas and putting on weight, reported a team of astronomers here 12 December at the Texas Symposium on Relativistic Astrophysics. If confirmed, the discovery would be the first evidence that black holes continue to grow long after their birth in the early universe.

Some of the best-studied black holes sit at the heart of quasars, which are among the brightest objects in the sky. As gas falls into these supermassive black holes, which weigh from 1 million to 1 billion times the mass of the sun, gravitational energy is released as light. But the amount of gas falling into these black holes is trivial compared with their mass, so scientists have long believed that the weight of the supermassive black holes changed little after their formation during the early universe.

Other black holes are hidden behind dust clouds, however, that obscure all but the highest energy x-rays. To find out whether these supermassive black holes are also dormant, a team of astronomers led by Amy Barger of the University of Hawaii turned their attention to a few hundred unusually bright nearby galaxies that they believe contained supermassive black holes. They pointed the Chandra X-ray Observatory at these galaxies and found that 20 of them, or about 10% of the sample, emitted high-energy x-rays. To produce so many dust-penetrating x-rays, the resident black holes must be chowing down on galactic gas, the team reports.

"They have done a beautiful job," says astrophysicist Andy Fabian of the Institute of Astronomy in Cambridge, U.K. But 20 galaxies aren't enough to convince Fabian and others that 10% of all supermassive black holes stay active and continue to eat up to the present day. Barger's team expects to glean more examples from Chandra observations scheduled during the next year.

--MARK SINCELL

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